A need exists to amplify signals comprising a single modulated carrier or a composite signal comprising several modulated carriers to high power levels for use in wireless communication base stations. The characteristics of the input signal require a high degree of linearity to substantially reduce and in some cases minimize distortion artifacts from appearing at the amplified output. In the RF frequency domain, standard high power amplifiers typically do not possess sufficient linearity to amplify either the single amplitude modulated RF signals nor the composite multiple RF carrier signals to meet stringent wireless base station requirements. Non-linear distortion products from such amplifiers can occur at the output in the form of spectral spreading or spectral regrowth of the modulated carriers or in spurious in band intermodulation products in the case of multiple RF carriers.
Currently, several techniques are used to compensate for high power amplifier non-linearity. For example, feedforward cancellation is a closed loop technique that introduces a compensation RF component into the output that cancels the nonlinear distortion products created by the amplifier. An example of a feedforward cancellation technique is described in U.S. Pat. No. 5,528,196, entitled "Linear RF Amplifier Having Reduced Intermodulation" (Baskum, et. al.), issued Jun. 18, 1996. Another technique is referred to as pre-distortion, which is a method to pre-distort the phase and amplitude of the input signal in a manner that counteracts and compensates for amplifier nonlinarity. The combination of the predistortion circuit and the nonlinear amplifier results in a net linear amplification. Examples of such an amplification technique are shown in U.S. Pat. No. 4,462,001, entitled "Baseband Linearizer for Wideband, High Power, Nonlinear Amplifiers" (Girard), issued Jul. 24, 1984 and U.S. Pat. No. 5,576,660, entitled "Broadband Predistortion Linearizer With Automatic Temperature Compensation For Microwave Amplifiers" (Pouysegur et. al.), issued Nov. 19, 1996.
Other prior art techniques are based on indirect methods of amplitude reconstruction. These methods generally separate the input signal into the amplitude and angle modulation components. The constant amplitude angle modulation component is easily amplified using nonlinear amplifiers. The amplitude modulation component is reintroduced into the output using one of several methods. In one method referred to as switched amplifiers, multiple power amplifiers are employed which are capable of being individually switched on and off to vary the power delivered to the load. In this method, the RF drive levels are switched on or off such that the number of active amplifiers is proportional to the signal envelope, or amplitude. When the multiple amplifier outputs are combined, the result is a signal whose envelope approximates that of the input signal. An example of this technique is shown in U.S. Pat. No. 5,132,637, entitled "RF Power Amplifier System Having Improved Distortion Reduction" (Swanson), issued Jul. 21, 1992. In another technique referred to as pulse modulation, the angle modulated component is further modulated with a pulse waveform prior to amplification. The pulse modulation frequency is selected to be much higher than the operating bandwidth of the input signal, and the duty cycle of the pulse waveform is adjusted to be proportional to the envelope modulation. The angle modulation which exists on the original signal is unaffected, but when the amplifier output is suitably band pass filtered, the original amplitude modulation is reintroduced onto the carrier waveform. An example of this technique is shown in U.S. Pat. No. 5,249,201, entitled "Transmission of Multiple Carrier Signals in a Nonlinear System" (Posner), issued Sep. 29, 1991. In a third technique referred to as phase modulation, a pair of high power amplifiers is required. The angle modulated carrier driving each amplifier is further separately and differentially phase modulated as a function of envelope signal. Phase modulation is introduced in a manner such that when the two amplifier output signals are combined, the imparted differential phasing causes the result to be amplitude modulated. An example of this technique is shown in U.S. Pat. No. 4,178,557, entitled "Linear Amplification With Nonlinear Devices" (P. Henry), issued Dec. 11, 1979.
In all of the latter three methods, a key element is the operation of the power amplifier(s) in a saturated or nearly saturated mode. This allows highly nonlinear amplifiers (such as Class C) to be used and also results in efficient power generation. In all cases, the result is linear amplification of the input signal. The degree of linear performance actually achieved generally depends on the precision of the implementation. Typically, linear dynamic range has been limited to 40 dB to 60 dB. Some systems, such as GSM, will require dynamic ranges on the order of 74 dB. Thus, a new approach is required to meet this requirement for very wide dynamic range.